This invention relates generally to turbomachinery and, more particularly, to the cooling of transition ducts that connect a plurality of combustors to the first stage of a gas turbine.
In a typical arrangement, combustors are arranged in an annular array within a compressor discharge opening, and are connected to the first stage of the turbine by transition ducts that are each shaped at one end to conform to a respective cylindrical combustor liner, and at an opposite end to conform to the turbine stage inlet. At the latter end, the transition duct has an external end frame by which the transition duct is secured to the turbine. In dry low NOx combustion systems in the assignee""s gas turbine product line, a perforated impingement cooling sleeve surrounds the transition duct, and is used to direct compressor discharge cooling air into contact with the transition duct. This cooling air eventually mixes with the fuel in the combustor.
Transition ducts and their associated impingement sleeves are packed together very tightly in the compressor discharge casing. As a result, there is little area through which the compressor discharge air can flow in order to cool the outboard part of the transition duct. Consequently, the air moves very rapidly through the narrow gaps between adjacent transition duct sidepanels, and the static pressure of the air is thus relatively low. Since impingement cooling relies on static pressure differential, the sidepanels of the transition ducts are therefore severely undercooled. As a result, the low cycle fatigue life of the ducts may be below that specified. An example of cooling transition ducts by impingement cooling may be found in commonly owned U.S. Pat. No. 4,719,748.
In accordance with an exemplary embodiment of this invention, poor cooling on the sidepanels of the transition duct is improved through the attachment of scoops on the external surface of the impingement sleeve, preferably along the side panels thereof, and thus also adjacent the side panels of the transition duct. These scoops stagnate the flow as it passes at high speed and redirects the flow onto the sidepanels of the transition duct. This redirect flow supplies sufficient cooling to cool the metal into a temperature zone where the material properties are sufficiently good to make the required life expectancy. The scoops may have various shapes and may be fixed to the sleeve in number and location determined by the shape of the sleeve, flow within the compressor discharge casing, and thermal loading on the transition duct.
In addition, the relative effectiveness of impingement cooling on non-scooped impingement cooling holes can be effected by the exact point of flow separation on the outboard side of the impingement sleeve relative to these cooling holes. Thus, another feature of this invention is to add aerodynamic devices to the surface of the impingement sleeve to enforce consistent flow separation at defined locations. In this regard, it has been noted in testing that locations on the edge of a hot sidepanel zone on the outboard side of the transition duct can see rapid and repeated fluctuations in temperature under certain load/geometric conditions. This instability in the temperature field is attributed to the separation point on the upper part of the impingement sleeve moving around in response to changes in compressor discharge swirl and other factors. A solution to this problem is to place features on the surface of the impingement sleeve that insure the flow separates at a given location relative to a cooling hole and thus produces a stable cooling flow. In the illustrative example, one or more solid members, such as, for example, a wire(s), is (are) fixed to the external surface of the impingement sleeve, extending generally along the sleeve outboard of a line of minimum space between adjacent impingement sleeves, and also outboard of the array of scoops arranged along the side panels of the impingement sleeve.
Accordingly, in one aspect, the present invention relates to a transition piece assembly for a gas turbine comprising a transition duct having one end adapted for connection to a gas turbine combustor and an opposite end adapted for connection to a first turbine stage, and a pair of side panels; an impingement sleeve surrounding the transition duct and establishing a cooling path therebetween, the impingement sleeve formed with a plurality of rows of cooling holes therein; and a plurality of flow catcher devices on an external surface of the impingement sleeve, each flow catcher at least partially surrounding one of the cooling holes.
In another aspect, the invention relates to a method of impingement cooling a transition duct connected between a gas turbine combustor and a first turbine stage with air discharged from a compressor comprising a) surrounding the transition duct with an impingement sleeve provided with a plurality of cooling holes; b) establishing a flow path for compressor discharge air along the impingement sleeve; and c) providing flow catcher devices on the impingement sleeve to catch and redirect the compressor discharge air through the impingement holes and onto the transition duct.